Sometimes, as happened in the summer of 2018, the less visible past reveals itself, just for a moment, and then quickly vanishes.

On very rare occasions, during unusually hot, dry summers, normally hidden features can appear on the landscape. This can occur in two ways. Areas where the remains of buildings lie just underground or where a stretch of land has been repeatedly walked upon are always drier. During a heat wave, vegetation there will wither more quickly, creating brown parch marks that contrast with the surrounding grassy areas. Most of the sites here follow this pattern. By contrast, more moisture collects in areas that in antiquity were taken up with ditches or were dug in other ways. In a heat wave, those areas will remain greener than the surrounding landscape. Such is the case with the Neolithic monument in Ireland’s Boyne Valley and the medieval castle in Wales.

As these conditions persisted this past summer, archaeologists, with the help of aerial photography, drones, and the eyes of the public and scholars alike, were able to document evidence of buildings and human behavior that have rarely, if ever, been seen before. The heat wave is over and the rain has come again. At least for now, many of these traces of the past are no longer visible.